Our previous results indicated that the expression of a calcium binding protein (CaBP) is coincident with the onset of calcium transport in the chorioallantoic membrane (CAM) of the chick embryo. We first propose to isolate and purify the CAM CaBP and then to characterize the physical properties of the protein. The molecular weight, isoelectric point, ion specificity, and calcium binding constant of the CaBP are of particular interest to (a) establish that the protein has the expected properties of a calcium binding protein, and (b) to compare the properties of the CAM and chick intestinal CaBP. Second, we propose to prepare antibodies against the purified CAM CaBP. By immunofluorescence techniques, we plan to determine the distribution of the CaBP within various cell types of the CAM ectoderm during embryonic development. Third, we propose to elucidate the effector(s) responsible for the development-dependent expression of the CAM CaBP and calcium transport by the addition of exogenous effectors to organ cultures of the CAM or injection of effectors into the embryo or the allantoic sac of the egg. By correlating the onset of calcium transport with the increase in CaBP activity in these latter experiments, the relationship between calcium transport and CaBP activity in the CAM can be established. The goal of this project is to define the mechanism and regulation of calcium transport by the CAM during embryonic skeletal formation. This information can be applied to our understanding of the mechanisms of calcium homeostasis and calcium mobilization across placenta-like membranes during embryogenesis.